Sister Superior has the most omph, being herald level with forcefields she can handle a lot of the opponents, and then also on your squad you've got Crimson Avenger, who can kill just about anything and is very hard to do much of anything to! (Only the magical swords might work on her)
Sorta-kinda. It's more, if she has data on someone, then she can mimic them exactly including a hologram. I'm not sure how she gets the fighting info though, she might need to spend time uploading it or such. So she can mimic Deathstroke flawlessly, but I'm not sure if she can pick up more on the fly.
She's most dangerous fighting as herself though. Powerful beam weaponry, area-effect sonics, high level super strength, point-defense forcefields, high level invulnerability...
No, but I can tell you what issue it was, JLA 100.
They set up a fight to fool Gaea, but to make it real, Vera was throwing attacks that left burnmarks and bruises on Clark, got her head bounced off the ground hard enough to break open the street, and so on.